This invention relates generally to two-way radio communication and more particularly to time division multiplexed digital communication and is more particularly directed to a communication system for the efficient utilization of the frequency spectrum.
Those skilled in the art will appreciate the congested and crowded nature of the available frequency spectrum. The Federal Communication Commission (FCC) have continually sought ways to reallocate the available spectrum or assign previously reserved spectrum to relieve this congestion. This condition is particularly noticeable in metropolitan areas where a large number of radio users are concentrated in a small geographic area. One proposal the FCC is considering is sharing a portion of the UHF television spectrum with the land mobile market (FCC docket 85-172). Another consideration is the reallocation of the land mobile reserve frequencies in the 896-902 MHz region to private land mobile uses (FCC docket 84-1233).
Another alternative for the FCC is to redefine the standard for land mobile communication channels. Currently, the standard for land mobile communication is a channel having a bandwidth of 25 kHz. However, the FCC may redefine this standard to use 12.5 kHz (or possibly 15 kHz) channels. The theory behind this "band-split" is to effectively double the number of channels in any newly allocated frequency spectrum. Potentially, as "older" spectrum is reallocated, all communications equipment will be required to operate in the 12.5 kHz channel bandwidth.
Although facially attractive, a band-split to double the available number of channels is not without cost. Present day communication devices operate with a sufficient frequency guard-band that protects against adjacent-channel interference (given the frequency stability of the transmitters). Of course, the band-split would also reduce the frequency guard-band tending to lead to higher adjacent-channel interference. Even assuming a greater than a two-to-one improvement in transmitter frequency stability, and high selectivity crystal filters for the receivers, adjacent-channel performance may be degraded by a band-split. Thus, there exists substantial technological barriers that must be overcome to provide a radio with comparable performance specifications at a competitive cost in the marketplace. Therefore, a substantial need exists in the market to develop a communication system that will provide an increase in the number of available communication channels that is compatible with present day 25 kHz channel bandwidths.